docs: def render_constants():
      """render generated constant files from templates"""
      generate_file("constant_enums.pxi", cython_enums, pjoin(root, 'zmq', 'backend', 'cython'))
      generate_file("constants.pxi", constants_pyx, pjoin(root, 'zmq', 'backend', 'cython'))
      generate_file("zmq_constants.h", ifndefs, pjoin(root, 'zmq', 'utils'))